West African Monsoon and Tropical Cyclones 1.Atlantic tropical cyclone variability: A climate...

West African Monsoon and Tropical Cyclones

1. Atlantic tropical cyclone variability: A climate perspective

2. Do African easterly waves matter?: A weather perspective

3. Future studies

1. Atlantic Tropical Cyclone Variability (ATCV)

There exists marked ATCV at interannual-to-decadal timescales.

What are the causes? Are they predictable?

1. Atlantic Tropical Cyclone Variability (ATCV)

Known factors:

Tropical Atlantic SSTs

ENSO

West African rainfall

Phase of QBO!!!

1. Atlantic Tropical Cyclone Variability (ATCV)

Known factors:

Tropical Atlantic SSTs

ENSO

West African rainfall

Phase of QBO!!!

1. Atlantic Tropical Cyclone Variability (ATCV)

Goldenberg and Shapiro (1996)

Linear correlation coefficients:

ENSO – ATCV -0.41

Sahel Rainfall – ATCV +0.70

Why is West Africa so important?

large-scale environmental impacts (e.g. shear)

weather systems (possibly)

1. Atlantic Tropical Cyclone Variability

vertical wind shear between 200mb and 925mb

1. Atlantic Tropical Cyclone Variability

Understanding the processes that influence the MDR shear and its variability is very important

West Africa and East Pacific both provide important anomalous heat sources that can impact the MDR shear through tropical teleconnections

1. Atlantic Tropical Cyclone Variability

Aiyyer and Thorncroft (2008) studied the variability of vertical shear and its causes in the 40 year ECMWF Reanalysis and other observations (SSTs, rainfall).

1. Atlantic Tropical Cyclone Variability

Shading shows percentage variance in shear explained

1. Atlantic Tropical Cyclone Variability

1. Atlantic Tropical Cyclone Variability

1. Atlantic Tropical Cyclone Variability

1. Atlantic Tropical Cyclone Variability

1. Atlantic Tropical Cyclone Variability

1. Atlantic Tropical Cyclone Variability

1. Atlantic Tropical Cyclone Variability

Thorncroft and Hodges (2001)

What about variability in the weather systems?

1. Atlantic Tropical Cyclone Variability

There is a hint that the number of strong vortices leaving the West African coast impacts ATCV but this is far from being a sure case.

Recent analysis in the ERA40 datset (Hopsch et al, 2006) suggests this relationship to be weak on interannual timescales - but not on interdecadal timescales!

1. Atlantic Tropical Cyclone Variability

Conclusions:

Variability in the large-scale environment in the tropical Atlantic strongly influences ATCV.

ATCV is influenced by ENSO and West African rainfall:

associated with changes in shear patterns in the MDR

subsidence?

ENSO is more important on interannual timescales

West Africa appears to be more important on multidecadal timescales

Variability in African easterly waves may also have some influence but is likely not to be as important as large-scale environmental factors.